The present invention relates, in general, to optical coupling of integrated circuits and, more particularly, to a system providing for the simultaneous bidirectional transfer of data between integrated circuits by means of modulated light beams.
Historically, interconnection between integrated circuits was limited to a two dimensional plane. Buses of parallel metal lines on a printed circuit board carried data from one circuit to another. Data to be transferred to adjacent boards in a system had to travel to the edge of the board, exit into a back plane, travel to the next board, and then to the destination integrated circuit. Simultaneous bidirectional communication was only possible if the width of the bus were doubled, doubling the complexity of the interconnection. Typical applications provided for bus arbitration to determine when the bus was clear, allowing data transmission in opposite directions.
Associated with each line was an inherent capacitance between the line and the ground plane, and an additional capacitance between adjacent lines. At relatively low clock speeds, capacitive loading was not a significant factor. However, as applications began to push clock speeds into the few hundred megahertz range and beyond, the inhibiting effects of the capacitance between interconnection lines and the ground plane, and of the capacitance between adjacent interconnection lines, became significant. Propagation delays due to capacitive loading effects became a limiting factor to circuit performance by limiting circuit speed and increasing circuit cross talk.
Additionally, wide data buses were plagued by simultaneous switching noise. Simultaneous switching noise was related to the parasitic inductances associated with power and ground interconnections to the bus. The level of the simultaneous switching noise was determined by the parasitic inductance, the width of the bus, and the rate of change of the drive current that charged up the interconnect lines. Finally, at relatively high frequencies, impedance matching terminations became necessary on interconnect lines to decrease signal settling time, thus adversely affecting the efficiency of data transfer due to increased power dissipation in the terminating impedance.
In order to overcome the limitations imposed by circuit interconnection, the use of optical interconnection was introduced. A number of optical interconnect approaches were advanced by Goodman, et. al., "Optical Interconnections for VLSI Systems", Proceedings of IEEE, vol 72, No.7, July 1984. One approach consisted of a number of opto-electronic transmitters, normally lasers, placed near the edge of an integrated circuit. The opto-electronic transmitters aimed beams of light at a holographic routing element located above the integrated circuit. The beams of light were modulated such that the beams of light contained the data to be transferred. The holographic routing element selectively diffracted the beams of light back to opto-electronic receivers on the surface of the integrated circuit.
Another approach, more typically used for a clock signal as opposed to data transfer, consisted of an opto-electronic transmitter located above an integrated circuit. Between the opto-electronic transmitter and the integrated circuit was located a holographic routing element. The opto-electronic transmitter emitted a modulated signal which was aimed by the holographic routing element onto opto-electronic receivers on the surface of the integrated circuit.
Still another approach utilized optical waveguides as described by Kapany and Burke, Optical Wave Guides, Academic Press, 1972. In this approach, the emissions of the opto-electronic transmitter were guided to an opto-electronic receiver by a fiber optic waveguide, a channel optical waveguide, or a planar optical waveguide.
Each of these approaches had limitations. The interface between the integrated circuit and the optical fiber was difficult to fabricate. This, and the physical size of the optical fiber, limited its use in many applications. Though channel waveguides were efficient for single data lines, buses were difficult to fabricate due to physical size constraints. The channel waveguide, as well as the planar optical waveguide, tended to be limited to a two dimensional plane. Conventional bussing was still required, and simultaneous bi-directional communication remained impractical.